1. Field of the Invention
The present invention relates to a working machine having an attachment that can swing around two axes of a horizontal axis and a vertical axis, and particularly relates to a front bracket that supports the attachment.
2. Description of the Related Art
FIG. 1 shows an example of such a working machine. The working machine shown is a small-type hydraulic shovel, and includes a lower propelling body 104, an upper slewing body 100 pivotably provided on the lower propelling body 104, and an attachment 101 fitted to a front part of the upper slewing body 100.
The attachment 101 has a bucket, an arm, a boom, and hydraulic cylinders. The attachment 101 is also supported by a front bracket 102 provided in the upper slewing body 100, via a swing bracket 103.
Specifically, a base end part of the attachment 101 is supported by the swing bracket 103 turnably around a horizontal axis HJ. The swing bracket 103 is supported by the front bracket 102 turnably around a vertical axis VJ. Accordingly, the hydraulic shovel shown can swing the attachment 101 to a right and left direction, in addition to being able to raise or lower the attachment 101 like a general hydraulic shovel.
Such structures of a swing bracket and a front bracket are disclosed in Japanese Unexamined Patent Application No. 2005-336920, Japanese Unexamined Patent Application No. 2005-16019, and Japanese Unexamined Patent Application No. 2002-146834, for example.
In general, a lower part of a rear part of the front bracket 102 is fixed to a bottom plate (not shown) pivotably supported by the lower propelling body 104. On the other hand, an upper part of the rear part of the front bracket 102 is welded to an upper end part of a vertical plate (not shown) stood on the bottom plate.
The working machine of this type has a problem in that stress tends to be concentrated to a welded portion of the front bracket 102, particularly, to a welded portion between the upper part of the rear part of the front bracket 102 and the vertical plate.
FIGS. 2A and 2B are enlarged views of portions indicated by an arrow X in FIG. 1.
When the attachment 101 is raised or lowered around the horizontal axis HJ, particularly when the attachment 101 is turned from a turned-up state (a virtual line) to a turned-down state (a solid line), strong load is applied to the front bracket 102 by the work of moment of force.
Specifically, the front bracket 102 is applied with force in a counterclockwise direction including force that presses the front bracket 102 upward, and force that stretches the front bracket 102 forward, as indicated by a void arrow in FIG. 2.
Further, when the attachment 101 is raised or lowered in a state that the attachment 101 is swung in a left or right direction, much stronger force is applied to the front bracket 102 in a left or right side.
Particularly, strong force is applied to the upper part of the front bracket 102.
Therefore, when the above force is applied to the front bracket 102, stress is concentrated to the welded portion of the front bracket 102, particularly, to the welded portion at the upper part of the rear part of the front bracket 102. Because the raise/lower operation of the attachment 101 is frequently performed, the stress concentration occurs repeatedly.
The upper part of the front bracket 102 is welded to an upper part of the vertical plate stood on the bottom plate. Because the upper part of the vertical plate is a free end, unlike a lower part of the vertical plate that is fixed to the bottom plate, strength and rigidity of the upper part of the vertical plate are structurally lower than strength and rigidity of the lower part of the vertical plate.
Consequently, when the attachment 101 is used for a long period, the welded portion of the front bracket 102 might be broken, and this is an important problem to be solved in securing durability of the working machine.